Issue #1668

Use it to prevent some unneeded state computation.

Issue #1653

Move hinting functions into it.

Since the addon is called merge, not diff.

Apparenly Firefox still hasn't caught up on the unprefixed version.

+ a few other fixes

Issue #1625

Issue #1625

Work around percentage width rounding problems.

Also makes sure that no two hinting dialogs can be active at the
same time.

Issue #1619

Use it to make xml-fold more robust, and matchbrackets much faster.

For folding Java and C/C++ style import/include blocks.